Packaging sheets in rigid and semi-rigid formats may be used for a variety of applications, ranging from fresh foods to medical devices. Certain applications include the use of the packaging at elevated temperatures. Examples of these applications include but are not limited to hot-filled food items and sterilization or cooking via steaming in a microwave oven, in these applications, the packaging materials may experience temperatures of from 70° C. to 100° C. or greater. For many of these applications, it is critical that the packaging sheet does not deform or lose considerable strength at the elevated temperatures.
Crystallized polyester (CPET) trays are commonly used for packaging that is subjected to high temperatures. These materials can withstand extreme temperatures, including oven cooking. CPET trays are typically used for frozen meals that can be reheated in a microwave oven or conventional oven. Due to the high levels of crystallinity in these materials, the thermoformed parts do not deform at elevated temperatures. However, CPET-based packaging material's are completely opaque, making it impossible to see the product contained in the package.
Polypropylene materials are also commonly used in packaging subjected to high temperatures. Retort processing (high temperature and pressure pasteurization completed after product is in the packaging) in polymeric packaging often uses polypropylene sealants because of their high heat resistance. Polypropylene materials may be chosen to optimize clarity. However, polypropylene can be difficult to thermoform, making it less attractive for rigid or semi-rigid applications.
Packaging sheets that use one amorphous polyester material solve many of the issues presented by CPET or polypropylene. An amorphous polyester can be processed to limit the crystallinity and to maintain high levels of clarity and low levels of haze for the packaging material. In addition, these materials thermoform easily (with fast cycle times, wide operating window, etc.). However, a sheet with one amorphous polyester packaging material typically does not have enough temperature resistance to hold its shape at elevated temperatures.